The study of low-mass stars and brown dwarfs

This thesis describes the search for brown dwarfs in open clusters using optical and infrared photometry, optical spectroscopy and surveys using the Hubble Space Telescope (HST). It also includes an observational study of very low-mass stars (VLMS) using a filter combination (I and Z), unused previously in this field enabling the calibration of the colour (I--Z), for future survey work.;Following a short introduction explaining the reasons for studying brown dwarfs this thesis reviews the theory of their formation and evolution and describes recent searches for brown dwarfs and their results. Modern CCD technology has led to larger and larger area cluster surveys and so chapter 3 reviews this technology and the data reduction software and techniques necessary for analysis.;Chapters 4, 5, 6 and 7 present the observational results. Chapter 4 presents an essentially null result of a deep HST survey in the Pleiades. Chapter 5 presents the d ata used to calibrate the I--Z colour and chapter 6 describes the discovery of PIZ 1, a brown dwarf in the Pleiades of mass 0.048 M⊙ and effective temperature ~2200K. Chapter 7 presents the infrared follow-up results of a number of recent large area optical surveys.;Chapters 8 and 9 use the results from the latest surveys to determine the mass and luminosity functions of both Praesepe and the Pleiades. In Praesepe the mass function is calculated down to the brown dwarf limit in the cluster and shows no sign of turning down. In the Pleiades the mass function appears to be flattening across the stellar sub-stellar boundary.;Finally in Chapter 10 I summarise the important conclusions from each chapter and identify areas of future work.